Velocity determination utilizing two photosensor arrays

ABSTRACT

A technique for relative velocity determination between a surface and a velocity determination system involves capturing a set of outputs from a first photosensor array and then comparing subsequently captured sets of outputs from a second photosensor array to the set of outputs from the first photosensor array until a satisfactory match is found between the outputs. Once a satisfactory match is found, the elapsed time between the capture of the two sets of outputs represents the time to travel the known separation distance between the two photosensor arrays. Given the known distance of travel and the elapsed time to travel the distance, the relative velocity is a simple calculation of the distance traveled divided by the time to travel the distance.

BACKGROUND OF THE INVENTION

In various circumstances it is desirable to know the relative velocitybetween a surface and an object. It is especially desirable in someapplications to be able to determine the relative velocity between asurface and an object without physical contact between the surface andthe object.

A prior art technique for determining the relative velocity between asurface and a velocity determination system involves capturing frames ofdigital image information with an image sensor and then performing across-correlation to identify the relative movement of an image featurethat is captured in both image frames. The relative movement of an imagefeature can then be used in combination with the elapsed time betweenframe capture to determine relative velocity. Although this techniqueworks well, the capturing of frames of digital image information and thecross-correlation of image features are both resource intensiveoperations.

SUMMARY OF THE INVENTION

A technique for relative velocity determination between a surface and avelocity determination system involves capturing a set of outputs from afirst photosensor array and then comparing subsequently captured sets ofoutputs from a second photosensor array to the set of outputs from thefirst photosensor array until a satisfactory match is found between theoutputs. Once a satisfactory match is found, the elapsed time betweenthe capture of the two sets of outputs represents the time to travel theknown separation distance between the two photosensor arrays. Given theknown distance of travel and the elapsed time to travel the distance,the relative velocity is a simple calculation of the distance traveleddivided by the time to travel the distance.

In an embodiment, determining whether two sets of outputs match eachother involves determining the difference between corresponding pixelson a pixel-by-pixel basis. The difference is then compared to adifference threshold and a satisfactory match is assumed to exist whenthe difference is less than the difference threshold.

Other aspects and advantages of the present invention will becomeapparent from the following detailed description, taken in conjunctionwith the accompanying drawings, illustrating by way of example theprinciples of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts an embodiment of a velocity determination system.

FIG. 2 depicts an example of two one-dimensional photosensor arrays witheight pixels each.

FIG. 3 depicts an example of two two-dimensional photosensor arrays ofsixty-four pixels each arranged in 8×8 matrices.

FIG. 4 illustrates two exemplary sets of pixel-specific outputs that aregenerated from the two photosensor arrays of FIG. 1 in the case whereeach photosensor array includes four pixels.

FIG. 5 illustrates two exemplary sets of pixel-specific outputs that aregenerated from the two photosensor arrays of FIG. 1 in the case wherethe photosensor arrays include eight pixels.

FIG. 6 is a process flow diagram of a method for relative velocitydetermination.

FIG. 7 depicts a more detailed process flow diagram of the relativevelocity calculation block from FIG. 6.

FIG. 8 depicts an embodiment of the velocity determination system fromFIG. 1 in which each photosensor module includes an illumination source.

FIG. 9 depicts a process flow diagram of a method for determining therelative velocity between a surface and a velocity determination system.

Throughout the description similar reference numbers may be used toidentify similar elements.

DETAILED DESCRIPTION

FIG. 1 depicts an embodiment of a velocity determination system 100 thatis positioned relative to a surface 102. Relative movement between thesurface and the velocity determination system may be caused by movementof the surface, movement of the velocity determination system, ormovement of both the surface and the velocity determination system. Thevelocity determination system includes two photosensor modules 104 and106 (identified as photosensor modules #1 and #2) and a processing unit110. Each photosensor module includes a photosensor array 114, 116(identified as photosensor arrays #1 and #2) and optics 124, 126. Eachphotosensor array includes an array of photosensitive pixels, referredto herein simply as “pixels.” Each pixel generates a pixel-specificoutput in response to detected light and each pixel-specific output isrepresentative of the intensity of light that is detected by therespective pixel. In an embodiment, the pixel-specific outputs areprovided as analog voltage or current signals that are representative ofthe intensity of the detected light. In another embodiment, thephotosensor outputs are converted to digital values and provided to theprocessor as digital values that are representative of the intensity ofthe detected light. For example, analog voltage or current signals areconverted to 7-bit digital values that are representative of theintensity of the detected light.

In an embodiment, the photosensor arrays 114, 116 are one-dimensionalarrays of pixels. FIG. 2 depicts an example of two one-dimensionalphotosensor arrays 114, 116 with eight pixels 130 each. In anotherembodiment, the photosensor arrays are two-dimensional arrays of pixels.FIG. 3 depicts an example of two two-dimensional photosensor arrays 114,116 of sixty-four pixels each arranged in 8×8 matrices. Exemplarydimensions of photosensor arrays include a 30×30 matrix of 50 um×50 umpixels and a 20×20 matrix of 10 um×10 um pixels. In the embodiment ofFIGS. 1-3, the two photosensor arrays of the velocity determinationsystem are identical. In particular, both photosensor arrays have thesame number of pixels in the same arrangement such that each pixel ofone photosensor array has a corresponding pixel in the other photosensorarray.

As indicated in FIGS. 1-3, the two photosensor arrays 114, 116 areseparated by a known separation distance, d. In an embodiment, theseparation distance is fixed within the velocity determination system100. The separation distance may be adjustable to different knowndistances and the particular separation distance isimplementation-specific.

In an embodiment, the two photosensor modules 104, 106 and theprocessing unit 110 of the velocity determination system 100 areenclosed within a housing 132. Structures within the housing set theseparation distance, d, between the photosensor arrays 114, 116 and alsoset the position of the photosensor modules within the housing.Additionally, the housing or some other structure (not shown) sets theposition of the housing relative to the surface 102 such that light 134reflected from the surface is detected by the photosensor arrays.Further, the photosensor arrays are positioned relative to the surfacesuch that both photosensor arrays eventually detect reflected light fromthe same location on the surface, although at different times.

The optics 124, 126 focus light, which is reflected off the surface 102,onto the respective photosensor array 114, 116. In an embodiment, theoptics of each photosensor module 104, 106 include a lens with a 1:1multiplying effect. In other embodiments, the optics may be selected andconfigured to expand or reduce the area of the surface that is coveredby each photosensor array.

The processing unit 110 is connected to receive the outputs from both ofthe photosensor arrays 114, 116. For example, the processing unit isconnected by parallel connections 144, 146 to each pixel of therespective photosensor arrays. Additionally, the processing unit couldbe connected to the photosensor arrays by serial connections. Theprocessing unit is configured to determine the relative velocity betweenthe surface and the velocity determination system in response to theoutputs that are received from the photosensor arrays. The techniqueimplemented by the processing unit to determine the relative velocity isdescribed in detail below. In an embodiment, the processing unit is anapplication-specific processor that is designed to determine relativevelocity as described below. In other embodiments, the processing unitmay be, for example, a multifunction microprocessor.

The basic operation of the velocity determination system 100 involvescapturing a set of outputs from a first photosensor array 114 (e.g.,photosensor array #1) and then comparing subsequently captured sets ofoutputs from a second photosensor array 116 (e.g., photosensor array #2)to the set of outputs from the first photosensor array until asatisfactory match is found between the outputs. A satisfactory matchbetween the outputs is a strong indication that the same location on thesurface 102 has been passed by both the first and second photosensorarrays. Once a satisfactory match is found, the elapsed time between thecapture of the two sets of outputs represents the time to travel theknown separation distance between the two photosensor arrays. Given theknown distance of travel and the elapsed time to travel the distance,the relative velocity is a simple calculation of the distance traveleddivided by the time to travel the distance.

A more detailed description of an embodiment of the velocitydetermination technique is now described. At a first time, t₁, a firstset of outputs is captured from the first photosensor array 114 (e.g.,photosensor array #1). At a second time, t₂, that is later than thefirst time, a second set of outputs is captured from the secondphotosensor array 116 (e.g., photosensor array #2). In both cases, thesets of outputs captured by the respective photosensor arrays include apixel-specific output from each pixel of the respective photosensorarray. FIG. 4 illustrates two exemplary sets of pixel-specific outputsthat are generated from the two photosensor arrays of FIG. 1 in the casewhere each photosensor array includes four pixels 130. As shown in FIG.4, each pixel generates a pixel-specific output that is independent ofthe other pixel-specific outputs in the array. Although FIG. 4illustrates photosensor arrays with four pixels, photosensor arrays withmore or less than four pixels are possible. FIG. 5 illustrates twoexemplary sets of pixel-specific outputs that are generated from the twophotosensor arrays of FIG. 1 in the case where the photosensor arraysinclude eight pixels. Because the photosensor arrays are positioned todetect light from the same location on the surface, it is expected thata set of outputs from one photosensor array will eventually match theset of outputs from the other photosensor array because the samelocation on the surface comes into view of both photosensor arrays,although at different times.

The number of pixels 130 in the photosensor arrays 114, 116 can impactthe accuracy of the velocity determination system 100. For example,increasing the number of pixels in each array can increase the accuracyof the matching processing, although the increased accuracy comes with acost of additional complexity.

Once the second set of outputs is captured, it is compared to the firstset of outputs to determine the difference between the two sets ofoutputs. In an embodiment, the difference is determined by comparing theoutputs from the two photosensor arrays 114, 116 on a pixel-by-pixelbasis. For example, the difference is determined according to thefollowing equation:

ΔS=(ΔS ₁ +ΔS ₂ +ΔS ₃ + . . . +ΔS _(n))/n  Eq. (1)

where;

ΔS₁=s1 ₁−s2 ₁;

ΔS₂=s1 ₂−s2 ₂;

ΔS₃=s1 ₃−s2 ₃;

ΔS_(n)=s1 _(n)−s2 _(n);

n=the total number of pixels in each photosensor array;

s1 _(x)=the signal from photosensor array #1, pixel x; and

s2 _(x)=the signal from photosensor array #2, pixel x, where x is thepixel number from 1 to n pixels.

In an embodiment, the difference, ΔS, is expressed as a percentdifference. For example, the difference is expressed in percentage termsas:

ΔS _(x) =[s1_(x) −s2_(x))/s1_(x)]*100;  Eq. (2) and

the cumulative percent difference between the outputs of the twodifferent photosensors is expressed as:

ΔS=(ΔS ₁ +ΔS ₂ +ΔS ₃ + . . . +ΔS _(n))/n.  Eq. (3)

Once the difference is determined, the difference is compared to apre-established difference threshold (DT). The difference threshold is avalue that is used to indicate whether there is a satisfactory matchbetween the two sets of outputs. In an ideal case, a differencethreshold of zero would indicate an exact match between the two sets ofoutputs. However, the photosensor arrays 114, 116 are not expected toperform ideally and setting a difference threshold to zero would beimpractical in most implementations. Therefore, the difference thresholdis set to a non-zero application-specific value that provides a strongindication that the same location on the surface 102 has been imaged byboth of the two photosensor arrays. In an embodiment, the differencethreshold can be set as a percentage difference between the two outputs.For example, a difference threshold of 2 percent could be establishedsuch that a satisfactory match is found if the percent difference isless than or equal to 2 percent and no match is found if the percentdifference is greater than or equal to two percent. An exemplarydifference calculation for a four pixel array is:

Pixel 1

s1₁ (base value): 100 s2₁: 99 ΔS₁ = [|(s1₁ − s2₁)|/s1₁] * 100: 1

Pixel 2

s1₂ (base value): 105 s2₂: 103 ΔS₂ = [|(s1₂ − s2₂)|/s1₂] * 100: 1.9048

Pixel 3

s1₃ (base value): 99 s2₃: 96 ΔS₃ = [|(s1₃ − s2₃)|/s1₃] * 100: 3.0303

Pixel 4

s1₄ (base value): 101 s2₄: 102 ΔS₄ = [|(s1₄ − s2₄)|/s1₄] * 100: 0.9901

Cumulative difference=ΔS=(ΔS ₁ +ΔS ₂ +ΔS ₃ +ΔS ₄)/4=1.7313.

If the determined difference is within the difference threshold (e.g.,ΔS is less than DT), then it is assumed that the sets of outputs fromthe two different photosensor arrays 114, 116 are satisfactorilymatched. On the contrary, if the determined difference exceeds thedifference threshold, then it is assumed that the sets of outputs do notmatch. In the above example, since the difference of 1.7313 percent isless than the difference threshold of 2 percent, a satisfactory match isfound.

Once it is determined that there is a match between two sets of outputs,then the elapsed time, T, between the capture of the set of outputs fromthe first photosensor array 114 and the capture of the set of outputsfrom the second photosensor array 116, is identified. With the elapsedtime, T, and the separation distance, d, both known, the relativevelocity between the surface and the velocity determination system iscalculated as v=d/T.

The accuracy of the velocity determination system is a function of themagnitude of the difference threshold. The particular optimal differencethreshold is dependent on the application. In an embodiment, increasingthe difference threshold too far can cause the accuracy of the velocitydetermination system to be reduced. Likewise, if the differencethreshold is too low, it becomes less likely that a satisfactory matchwill be found and therefore no velocity determination can be made. In anembodiment, the processing unit includes a programmable register and aprogramming interface for setting the difference threshold. Including aprogrammable register to set the difference threshold enables thevelocity determination system to be configured to accommodate surfaceswith imagable features of different quality.

In operation, the capture of the sets of outputs from the photosensorarrays and the processing of the captured outputs is controlled toeffectively determine the relative velocity between the surface and thevelocity determination system. An exemplary process flow diagram of atechnique for relative velocity determination is described withreference to FIGS. 6 and 7. Referring to FIG. 6, after system power up,block 200, the photosensor arrays (e.g., photosensor arrays #1 and #2)are reset, block 202. For example, resetting the photosensor arraysinvolves charging or discharging each pixel to a certain known voltagelevel. At decision point 204, it is determined whether the output fromphotosensor array #1 at time t₁, S1(t ₁), is greater than zero. If theoutput is not greater than zero, then the process returns to block 202.If the output is greater than zero, then a timer is started, block 206.At decision point 208, it is determined if a timeout has occurred. In anembodiment, a timeout is introduced to check for errors. The length ofthe timeout interval is dependent on the application. If a timeoutoccurs, the process returns to block 206 and if a timeout has notoccurred, then the output, S1(t ₁), is stored and/or recorded, block210. At decision point 212, it is determined whether the output fromphotosensor array #2 at time t₂, S2(t ₂), is greater than zero. If theoutput is not greater than zero, then the process returns to block 202.If the output is greater than zero, then the output, S2(t ₂), is storedand/or recorded, block 214. At block 216, the relative velocity isdetermined using the outputs S1(t ₁) and S2(t ₂) and the knownseparation distance, d. A more detailed description of the operationsassociated with block 216 is provided with reference to FIG. 7. Once therelative velocity is determined, at decision point 218 it is determinedif there is an error or a timeout. If there is an error or a timeout,the process returns to block 202. If there is no error or timeout, thenthe determined relative velocity is displayed, stored, and/or used insome other manner, block 220.

FIG. 7 depicts a more detailed process flow diagram of the relativevelocity calculation that occurs once two sets of outputs are captured.At block 300, the difference between the outputs S1(t ₁) and S2(t ₂) isdetermined. In particular, the difference is calculated as ΔS(t)=S1(t₁)−S2(t ₂), for example, using eq. (1). At decision point 302, thedifference is compared to a pre-established difference threshold, DT. Ifthe difference, ΔS(t), is greater than the difference threshold, DT,then there is not a satisfactory match and the process returns to blockX to wait for new sets of outputs. If the difference, ΔS(t), is notgreater than the difference threshold, DT, then there is a satisfactorymatch, block 304. At block 306, the elapsed time, T, between the captureof the two sets of outputs is identified and stored and/or recorded. Inthis example, the elapsed time, T, is expressed as T=t₂−t₁. At block308, the relative velocity is calculated as v=d/T, where d is theseparation distance.

In an embodiment, each photosensor module includes an illuminationsource configured to illuminate a spot on the surface. For example, theillumination sources are positioned so that a large portion of thereflected light is directed towards the corresponding photosensor array.FIG. 8 depicts an embodiment of the velocity detection system 100 fromFIG. 1 in which each photosensor module 104, 106 includes anillumination source 164, 166. In an alternative embodiment, a singleillumination source may be used to illuminate the surface to providereflected light for both of the two photosensor arrays.

FIG. 9 depicts a process flow diagram of a method for determining therelative velocity between a surface and a velocity determination system.At block 400, at a first time, a set of outputs is captured from a firstphotosensor array, wherein the first photosensor array comprises anarray of pixels, with each pixel generating a pixel-specific output inresponse to light reflected from the surface, the pixel-specific outputsbeing representative of the intensity of the reflected light. At block402, at a second time that is later than the first time, a set ofoutputs is captured from a second photosensor array, wherein the secondphotosensor array comprises an array of pixels, with each pixelgenerating a pixel-specific output in response to light reflected fromthe surface, the pixel-specific outputs being representative of theintensity of the reflected light, wherein the first and secondphotosensor arrays are separated by a known separation distance. Atblock 404, the difference between the set of outputs from the firstphotosensor array and the set of outputs from the second photosensorarray is determined. At block 406, the difference between the set ofoutputs from the first photosensor array and the set of outputs from thesecond photosensor array are compared to a pre-established differencethreshold. At block 408, the elapsed time between the capture of the setof outputs from the first photosensor array and the capture of the setof outputs from the second photosensor array is identified when thecomparison of the difference between the sets indicates that thedifference is within the pre-established difference threshold. At block410, the relative velocity between the surface and the velocitydetection system is calculated using the elapsed time and the knownseparation distance.

Although one technique for determining if a satisfactory match existsbetween two sets of outputs, other techniques for determining if asatisfactory match exists between two sets of outputs can be usedwithout deviating from the scope of the invention. For example, onetechnique for comparing the outputs from the two photosensor arrays mayinvolve a convolution. In an embodiment, the outputs from the twophotosensor arrays are defined as functions f and g, respectively, andif f*g=1, then the two outputs are considered a match. Another techniqueinvolves finding the average value for the output from each photosensorarray and then comparing the two average values. This technique can beexpressed, for example, as:

S1_(ave)=(s1₁ +s1₂ +s1₃ + . . . +s1_(n))/n;

S2_(ave)=(s2₁ +s2₂ +s2₃ + . . . +s2_(n))/n;

-   -   where:        -   S1 _(ave)=the average signal from photosensor #1;        -   S2 _(ave)=the average signal from photosensor #2; and    -   n=the number of pixels in each photosensor.        such that a match is found if S1 _(ave)=S2 _(ave).

In an embodiment, the two photosensor arrays 114 and 116 are identifiedsub-arrays of pixels within a large pixel array. That is, a large pixelarray is fabricated on a single substrate and the two photosensor arrays114 and 116 are two separate sub-arrays within the large pixel array.Although the two separate sub-arrays are within the same large pixelarray, they are functionally identified as two separate photosensorarrays. Using this configuration, the velocity determination proceeds asdescribed above.

Although specific embodiments of the invention have been described andillustrated, the invention is not to be limited to the specific forms orarrangements of parts as described and illustrated herein. The inventionis limited only by the claims.

1. A method for determining relative velocity between a surface and avelocity detection system, the method comprising: at a first time,capturing a set of outputs from a first photosensor array of thevelocity detection system, wherein the first photosensor array comprisesan array of pixels, with each pixel generating a pixel-specific outputin response to light reflected from the surface, the pixel-specificoutputs being representative of the intensity of the reflected light; ata second time that is later than the first time, capturing a set ofoutputs from a second photosensor array of the velocity detectionsystem, wherein the second photosensor array comprises an array ofpixels, with each pixel generating a pixel-specific output in responseto light reflected from the surface, the pixel-specific outputs beingrepresentative of the intensity of the reflected light, wherein thefirst and second photosensor arrays are separated by a known separationdistance; determining the difference between the set of outputs from thefirst photosensor array and the set of outputs from the secondphotosensor array; comparing the difference between the set of outputsfrom the first photosensor array and the set of outputs from the secondphotosensor array to a pre-established difference threshold; identifyingthe elapsed time between the capture of the set of outputs from thefirst photosensor array and the capture of the set of outputs from thesecond photosensor array when the comparison of the difference betweenthe sets indicates that the difference is within the pre-establisheddifference threshold; and calculating the relative velocity between thesurface and the velocity detection system using the elapsed time and theknown separation distance.
 2. The method of claim 1 wherein determiningthe difference between the set of outputs comprises determining thedifference of the pixel-specific outputs on a pixel-by-pixel basisbetween the first and second photosensor arrays.
 3. The method of claim2 wherein the first and second photosensor arrays have correspondingpixels.
 4. The method of claim 3 wherein the differences of thepixel-specific outputs are determined for corresponding pixels.
 5. Themethod of claim 2 wherein the first and second photosensor arrays areidentical to each other.
 6. The method of claim 2 wherein the first andsecond photosensor arrays have the same number of pixels in the samearrangement.
 7. The method of claim 1 wherein comparing the differencebetween the set of outputs from the first photosensor array and the setof outputs from the second photosensor array to a pre-establisheddifference threshold comprises comparing the difference between outputsfrom the first photosensor array and outputs from the second photosensorarray to a pre-established difference threshold on a pixel-by-pixelbasis.
 8. The method of claim 1 further comprising adjusting thepre-established difference threshold.
 9. A velocity determination systemfor determining relative velocity between a surface and the velocitydetection system, the velocity determination system comprising: a firstphotosensor array comprising an array of pixels, wherein each pixel isconfigured to generate a pixel-specific output in response to lightreflected from the surface, the pixel-specific outputs beingrepresentative of the intensity of the reflected light; a secondphotosensor array comprising an array of pixels, wherein each pixel isconfigured to generate a pixel-specific output in response to lightreflected from the surface, the pixel-specific outputs beingrepresentative of the intensity of the reflected light; wherein thefirst and second photosensor arrays are separated by a known separationdistance; and a processing unit configured to: determine the differencebetween a set of outputs captured from the first photosensor array at afirst time and a set of outputs captured from the second photosensorarray at a second time that is later than the first time; compare thedifference between the set of outputs from the first photosensor arrayand the set of outputs from the second photosensor array to apre-established difference threshold; identify the elapsed time betweenthe capture of the set of outputs from the first photosensor array andthe capture of the set of outputs from the second photosensor array whenthe comparison of the difference between the sets indicates that thedifference is within the pre-established difference threshold; andcalculate the relative velocity between the surface and the velocitydetection system using the elapsed time and the known separationdistance.
 10. The velocity determination system of claim 9 wherein theprocessing unit is configured such that determining the differencecomprises determining the difference of the pixel-specific outputs on apixel-by-pixel basis between the first and second photosensor arrays.11. The velocity determination system of claim 10 wherein the first andsecond photosensor arrays have corresponding pixels.
 12. The velocitydetermination system of claim 11 wherein the differences of thepixel-specific outputs are determined for corresponding pixels.
 13. Thevelocity determination system of claim 10 wherein the first and secondphotosensor arrays are identical to each other.
 14. The velocitydetermination system of claim 10 wherein the first and secondphotosensor arrays have the same number of pixels in the samearrangement.
 15. The velocity determination system of claim 9 whereincomparing the difference between the set of outputs from the firstphotosensor array and the set of outputs from the second photosensorarray to a pre-established difference threshold comprises comparing thedifference between outputs from the first photosensor array and outputsfrom the second photosensor array to a pre-established differencethreshold on a pixel-by-pixel basis.
 16. The velocity determinationsystem of claim 9 wherein the first and second photosensor arrays areone-dimensional arrays.
 17. The velocity determination system of claim 9wherein the first and second photosensor arrays are two-dimensionalarrays.
 18. The velocity determination system of claim 9 wherein theprocessing unit comprises means for adjusting the pre-establisheddifference threshold.
 19. The velocity determination system of claim 9further comprising parallel connections between each pixel of thephotosensor arrays and the processing unit.
 20. A method for determiningrelative velocity between a surface and a velocity detection system, themethod comprising: at a first time, capturing a set of outputs from afirst photosensor array of the velocity detection system, wherein thefirst photosensor array comprises an array of pixels, with each pixelgenerating a pixel-specific output in response to light reflected fromthe surface, the pixel-specific outputs being representative of theintensity of the reflected light; at a second time that is later thanthe first time, capturing a set of outputs from a second photosensorarray of the velocity detection system, wherein the second photosensorarray comprises an array of pixels, with each pixel generating apixel-specific output in response to light reflected from the surface,the pixel-specific outputs being representative of the intensity of thereflected light, wherein the first and second photosensor arrays areseparated by a known separation distance; determining if there is asatisfactory match between the set of outputs from the first photosensorarray and the set of outputs from the second photosensor array;identifying the elapsed time between the capture of the set of outputsfrom the first photosensor array and the capture of the set of outputsfrom the second photosensor array when a satisfactory match isdetermined; and calculating the relative velocity between the surfaceand the velocity detection system using the elapsed time and the knownseparation distance.